Rename itango script to itango3 for python3

This PR should solve the file conflicts when installing both python 2 and python 3 version of PyTango.

It adds a bit of code duplication but it should be removed later on anyway by using entry points instead of scripts. Another issue is that it might not be obvious that itango3 refers to python 3 and not tango 3.

This PR also updates the boost library name for python3 on Fedora and CentOS.

Still not completely tested, will confirm soon.